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Invalid use of = operator

Assignment in conditional statement

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Description

This defect occurs when an assignment is made inside the predicate of a conditional, such as if or while.

In C and C++, a single equal sign is an assignment not a comparison. Using a single equal sign in a conditional statement can indicate a typo or a mistake.

Polyspace® does not report an Invalid use of = (assignment) operator when the object being assigned is declared in the same statement, such as if statements with initializer. For example, in this code snippet, the variable tol is declared and assigned the return value of the function tolerance in the if statement, but Polyspace does not report the use of the = operator as a violation.

#include <iostream>
#include <stdexcept>
#include <string>

extern float tolerance(float actual, float expected);
enum STATUS
{
  FAIL,
  PASS
};

STATUS func(float val, float size)
{
  if (auto tol = tolerance(val, size) < 0.01f) /* No defect. Equivalent to
  
                                                auto tol = tolerance(val, size);
                                                if( tol < 0.01f) */
  {
    return PASS;
  }
  else
  {
    std::string errorMsg = "Tolerance exceeded by " + std::to_string(tol - 0.01f);
    throw std::runtime_error(errorMsg);
  }
}

Risk

  • Conditional statement tests the wrong values— The single equal sign operation assigns the value of the right operand to the left operand. Then, because this assignment is inside the predicate of a conditional, the program checks whether the new value of the left operand is nonzero or not NULL.

  • Maintenance and readability issues — Even if the assignment is intended, someone reading or updating the code can misinterpret the assignment as an equality comparison instead of an assignment.

Fix

Examples

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#include <stdio.h>

void bad_equals_ex(int alpha, int beta)
{
    if(alpha = beta)
    {
        printf("Equal\n");
    }
}

The equal sign is flagged as a defect because the assignment operator is used within the predicate of the if-statement. The predicate assigns the value beta to alpha, then implicitly tests whether alpha is true or false.

Correction — Change Expression to Comparison

One possible correction is adding an additional equal sign. This correction changes the assignment to a comparison. The if condition compares whether alpha and beta are equal.

#include <stdio.h>

void equality_test(int alpha, int beta)
{
    if(alpha == beta)
    {
        printf("Equal\n");
    }
}
Correction — Assignment and Comparison Inside the if Condition

If an assignment must be made inside the predicate, a possible correction is adding an explicit comparison. This correction assigns the value of beta to alpha, then explicitly checks whether alpha is nonzero. The code is clearer.

#include <stdio.h>

int assignment_not_zero(int alpha, int beta)
{
    if((alpha = beta) != 0)
    {
        return alpha;
    }
    else
    {
        return 0;
    }
}
Correction — Move Assignment Outside the if Statement

If the assignment can be made outside the control statement, one possible correction is to separate the assignment and comparison. This correction assigns the value of beta to alpha before the if. Inside the if-condition, only alpha is given to test if alpha is nonzero or not NULL.

#include <stdio.h>

void assign_and_print(int alpha, int beta)
{
    alpha = beta;
    if(alpha)
    {
        printf("%d", alpha);
    }
}

Result Information

Group: Programming
Language: C | C++
Default: On for handwritten code, off for generated code
Command-Line Syntax: BAD_EQUAL_USE
Impact: Medium

Version History

Introduced in R2013b

See Also

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